(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of preventing latchup and decreasing chip size in the manufacture of integrated circuits.
(2) Description of the Prior Art
In the fabrication of integrated circuit devices, it is desired to decrease chip size and increase device density. A problem that may occur is a difference in the threshold voltage between the source and the body, or gate, known as the body effect. An increase in the body effect can lead to latchup. Latchup is the condition in which a very low resistance path is established between the V.sup.DD and V.sup.SS power lines, allowing large currents to flow through the circuit. This may cause device malfunction or even destruction due to heat damage. For example, latchup is discussed in the book Silicon Processing for the VLSI Era, Volume 2, by Stanley Wolf, Lattice Press, Sunset Beach, Calif., c. 1990, pp. 400-419.
The bulk CMOS structure has an inherent pnp and npn parasitic bipolar junction transistor. Under certain conditions, such as overshoot of voltage, the lateral current in the well and substrate can cause a voltage drop, hence a forward bias in some junctions, causing a large current flow. Based on the theory, if the bulk of the devices can be fully separated from one another, this phenomena can be avoided.
U.S. Pat. No. 5,441,900 to Bulucea et al discloses ion implantation to suppress latchup. U.S. Pat. No. 5,451,534 to Yang teaches forming a gate electrode by ion implantation into a silicon substrate and source and drain regions formed in an overlying polysilicon layer. A similar thin film transistor is shown in FIG. 3. 23 in Principles of CMOS VLSI Design, by Weste and Eshraghian, Addison-Wesley Publishing Company, Reading, Mass., c. 1993, pp. 139-140.